The rack of a vehicle rack and pinion steering system has wide gear teeth and a round rack and rack bearing interface.
The loads on the rack of a rack and pinion steering system are applied by one or two tie rods and a driven pinion in both manual steering systems and in electric power steering systems. Each tie rod transmits force from the rack to steered wheels and transmits force from a road surface to the rack. The forces transmitted to and from the rack by the tie rods are generally applied at an angle relative to the rack axis. In steering systems with tie rods attached to each end of a rack, the force applied to one rack end is not the same as the force applied to the other rack end in either magnitude or direction. These forces tend to rotate the rack about the rack axis. The forces also tend to move the rack and rack axis out of alignment with a rack housing axis.
The racks of rack and pinion steering systems have generally been either round or V-shaped. Round racks have teeth formed in a line along one side of an elongated cylindrical member. A bearing with a cylindrical bearing surface supports a rack with a circular cross section. Racks with a circular cross section supported by a bearing surface having a constant radius minimize friction and allow the rack to instantaneously reach equilibrium during pinion rotation. Friction loads on the rack and noise are both minimized. However a circular rack has narrow teeth with minimal load carrying capacity. The load carrying capacity can be increased by increasing the diameter of a round rod. A rod with an increased diameter has an increased weight and increased cost.
Racks with a V-shape have wider teeth. Wider teeth have increased load carrying capacity for a given rod cross section area. The loads exerted upon the V-shaped surfaces by rack support bearings due to rack rotation tend to be concentrated thereby increasing wear. There is also lash between the rack and the V-shaped bearing. This lash results in noise when the loads tending to rotate the rack are reversed.
The vehicle rack and pinion steering system includes an elongated rack bar having a first end and a second end. A first tie rod connector is on the first end and a second tie rod connector is on the second end. A rack axis passes through the first end and the second end. At least one bearing contact surface on the elongated bar extends at least a portion of the distance from the first end to the second end. The at least one bearing contact surface is an arc about the rack axis and has a substantially fixed radius. A plurality of rack gear teeth are integral with the elongated rack bar and are on the opposite side of the rack axis from the at least one bearing contact surface. The width of the plurality of rack gear teeth, transverse to the rack axis, is at least twice the substantially fixed radius.